1. Field of the Invention
The present invention relates to white balance correction and, more particularly, to a white balance correction circuit for use in a negative-to-positive conversion apparatus which shoots images recorded on a negative recording medium, and as a color negative film, to produce a television signal representative of positive images associated therewith.
2. Description of the Prior Art
Generally, color negative films are exposed in various lighting conditions, or light sources, and the color temperature of a light source is rarely corrected at the time of shooting. Correction of the color temperature is usually effected by a printer when a positive picture is reproduced from the negative picture on a sheet of sensitized paper.
When a picture recorded on a color negative is to be picked up by a television (TV) camera to be converted into a TV signal, simple negative-to-positive (NP) conversion process for merely inverting negative image into a positive image is insufficient because the color temperature has to be corrected as well.
To correct color temperatures under various condition, it is a primary requisite that correction be applied to at least two different groups of colors. Such a requisite has heretofore been implemented by, for example, an arrangement which allows one to correct color temperature by manipulating knobs assigned to a group of red and blue and a group of green and purple, while observing a picture displayed on a cathode ray tube (CRT) color monitor. However, adjusting the two different groups of colors while looking at the display requires expertness because it is difficult to adjust two factors on an orthogonal plane, that is, it is impossible to visually recognize the origin of each of red-blue and green-purple color vectors. While a vectorscope may be employed to attain accurate adjustment, adjustment relying on such a measuring instrument is unfeasible for use at home.
In a telecine system for broadcasting applications, it is a usual practice to use films which were exposed under standardized lighting and, therefore, the adjustment of white balance is relatively easy. However, the situation concerned with white balance adjustment is quite severe at odinary home; various negative films exposed under arbitrary or non-standardized lighting have to be picked up under other lighting conditions to be produced in the form of TV signals. In a strict sense, it is impractical to faithfully reconstruct or reproduce information representative of an original shooting field from negative information which has been partly lost. Nevertheless, in the broadcasting telecine system or the like, it has been performed by an expert skilled in TV camera adjustment to reproduce a picture on a monitor with its color balance and highlight and shadow portions corrected to an acceptably natural appearance, based only on a relationship between negative picture information and lighting for reproduction.
At home, however, such a high level of adjustment aided by experience cannot be expected for reproduction. Additionally, a mechanism associated with the device for the adjustment is incapable of performing accurate adjustment, while the use of a measuring instrument is impractical from the standpoint of cost-effective construction or the ease of handling.
Meanwhile, color negative films which are widespread today have wider latitudes so that pictures recorded thereon significantly differ from each other in the amount of exposure as well as in the deviation from adequate color balance. As well known, positive color prints are produced from such various color negative films at a special laboratory while optimumly correcting colors and the like. In TV playback system, however, it has become increasingly difficult to correct luminance and colors or at least to adjust white balance itself. The irregularity in the amount of exposure may be corrected to a noticeable extent by, for example, adjusting an iris in an imaging section of an NP conversion system. However, depending upon the object, a picture having a substantial luminance difference measured by the whole area photometry of a shooting field, for example, cannot be sufficiently corrected by iris adjustment alone.